This invention relates to air-flow-rate sensors for detecting air-flow rate more or less than a predetermined level which are used, for example, for detecting ventilation failure, and in particular, to those air-flow-rate sensors including a thermo-magnetically operated switch and a heater disposed adjacent the switch.
Air-flow-rate sensors have been used in various fields where it is required to detect whether the current air-flow rate is more than a predetermined level or not. For example, the sensors have been used for detecting the ventilation failure in electronic or electric equipments or a forced air-cooling type, and controlling alarm devices or power feed to those equipments so as to protect those equipments from undesirable overheating due to the ventilation failure.
A known air-flow-rate sensor, which is particularly adaptable for detecting the ventilation failure, includes a thermo-magnetically operated switch and an electric heater disposed adjacent the switch. The thermo-magnetically operated switch is formed by an assembly of a reed switch, one or more temperature sensitive ferromagnetic bodies and one or more permanent magnets, as disclosed in French Pat. No. 1,549,349, U.S. Pat. No. 3,750,064 and others. The heater uses a heating wire such as a nichrome wire or a metal oxide blazed resistor as a heating element.
In use, the sensor having the thermo-magnetically operated switch is disposed in an air flow to be monitored, for example, mounted to a ventilation fan, and the heater is energized. During a period when the air flow is maintained above a predetermined rate, the heater and the thermo-magnetically operated switch are air-cooled and therefore, the temperature sensitive ferromagnetic body is not heated to its Curie point so that the thermo-magnetically operated switch does not operate. When air flow stops, or when air-flow rate considerably reduces, due to a certain trouble, for example, breakdown of the ventilation fan, the temperature sensitive ferromagnetic body is heated above its Curie point so that the thermo-magnetically operated switch operates, that is the ventilation failure is detected.
Although the known air-flow-rate sensor using the thermo-magnetically operated switch is excellent in the life time and reliability, it has several problems for safety. During the ventilation failure period, the heater temperature elevates abnormally to cause a fire. Since the sensor is covered with duct in long use, a fire becomes apt to be caused by overheating. Furthermore, dust accumulating in a gap between the heater and the thermo-magnetically operated switch interferes heat transfer from the heater to the thermo-magnetically operated switch, so that the responsibility of the sensor is degraded. This also results to overheating of the electric equipment of a forced air-cooling type.